Summary of Heart Failure
Congestive heart failure (CHF) is a generally progressive, life threatening condition in which myocardial contractility is depressed such that the heart is unable to adequately pump the blood returning to it, also referred to as decompensation. Symptoms include breathlessness, fatigue, weakness, leg swelling, and exercise intolerance. On physical examination, patients with heart failure often have elevated heart and respiratory rates (an indication of fluid in the lungs), edema, jugular venous distension, and enlarged hearts. The most common cause of CHF is atherosclerosis, which causes blockages in the coronary arteries that provide blood flow to the heart muscle. Ultimately, such blockages may cause myocardial infarction with subsequent decline in heart function and resultant heart failure. Other causes of CHF include valvular heart disease, hypertension, viral infections of the heart, alcohol consumption, and diabetes. Some cases of CHF occur without clear etiology and are called idiopathic. The effects of CHF on a subject experiencing the condition can be fatal.
There are several types of CHF. Two types of CHF are identified according to which phase of the cardiac pumping cycle is more affected. Systolic heart failure occurs when the heart's ability to contract decreases. The heart cannot pump with enough force to push a sufficient amount of blood into the circulation leading to a reduced left ventricular ejection fraction. Lung congestion is a typical symptom of systolic heart failure. Diastolic heart failure refers to the heart's inability to relax between contractions and allow enough blood to enter the ventricles. Higher filling pressures are required to maintain cardiac output, but contractility as measured by left ventricular ejection fraction is typically normal. Swelling (edema) in the abdomen and legs is a typical symptom of diastolic heart failure. Often, an individual experiencing heart failure will have some degree of both systolic heart failure and diastolic heart failure.
CHF is also classified according to its severity. The New York Heart Association classifies CHF into four classes: Class I involves no obvious symptoms, with no limitations on physical activity; Class II involves some symptoms during or after normal activity, with mild physical activity limitations; Class III involves symptoms with less than ordinary activity, with moderate to significant physical activity limitations; and Class IV involves significant symptoms at rest, with severe to total physical activity limitations. Typically, an individual progresses through the classes as they live with the condition.
Although CHF is generally thought of as a chronic, progressive condition, it can also develop suddenly. This type of CHF is called acute CHF, and it is a medical emergency. Acute CHF can be caused by acute myocardial injury that affects either myocardial performance, such as myocardial infarction, or valvular/chamber integrity, such as mitral regurgitation or ventricular septal rupture, which leads to an acute rise in left ventricular and diastolic pressure resulting in pulmonary edema and dyspnea.
Common treatment agents for CHF include vasodilators (drugs that dilate blood vessels), positive inotropes (drugs that increase the heart's ability to contract), and diuretics (drugs to reduce fluid). Additionally, beta-antagonists (drugs that antagonize beta-adrenergic receptors) have become standard agents for treating mild to moderate heart failure. Lowes et al, Clin. Cardiol., 23:III11-6 (2000).
Positive inotropic agents include beta-adrenergic agonists, such as dopamine, dobutamine, dopexamine, and isoproterenol. However, use of a beta-agonist has potential complications, such as arrhythmogenesis and increased oxygen demand by the heart. Additionally, the initial short-lived improvement of myocardial contractility afforded by these drugs is followed by an accelerated mortality rate resulting largely from a greater frequency of sudden death. Katz, HEART FAILURE: PATHOPHYSIOLOGY, MOLECULAR BIOLOGY AND CLINICAL MANAGEMENT, Lippincott, Williams & Wilkins (1999).
Beta-antagonists antagonize beta-adrenergic receptor function. While initially contra-indicated in heart failure, they have been found to provide a marked reduction in mortality and morbidity in clinical trials. Bouzamondo et al., Fundam. Clin. Pharmacol., 15: 95-109 (2001). Accordingly, they have become an established therapy for heart failure. However, even subjects that improve under beta-antagonist therapy may subsequently decompensate and require acute treatment with a positive inotropic agent. Unfortunately, as their name suggests, beta-antagonists block the mechanism of action of the positive inotropic beta-agonists that are used in emergency care centers. Bristow et al., J. Card. Fail., 7: 8-12 (2001).
Vasodilators, such as nitroglycerin, have been used for a long period of time to treat heart failure. However, the cause of nitroglycerin's therapeutic effect was not known until late in the last century when it was discovered that the nitric oxide molecule (NO) was responsible for nitroglycerin's beneficial effects. In some subjects experiencing heart failure, a nitric oxide donor is administered in combination with a positive inotropic agent to both cause vasodilation and to increase myocardial contractility. However, this combined administration can impair the effectiveness of positive inotropic treatment agents. For example, Hart et al, Am. J. Physiol. Heart Circ. Physiol., 281:146-54 (2001) reported that administration of the nitric oxide donor sodium nitroprusside, in combination with the positive inotropic, beta-adrenergic agonist dobutamine, impaired the positive inotropic effect of dobutamine. Hare et al., Circulation, 92:2198-203 (1995) also disclosed the inhibitory effect of nitric oxide on the effectiveness of dobutamine.
As described in U.S. Pat. No. 6,936,639, compounds that donate nitroxyl (HNO) under physiological conditions have both positive inotropic and lusitropic effects and offer significant advantages over existing treatments for failing hearts. Due to their concomitant positive inotropic/lusotropic action and unloading effects, nitroxyl donors were reported as helpful in treating cardiovascular diseases characterized by high resistive load and poor contractile performance. In particular, nitroxyl-donating compounds were reported as useful in the treatment of heart failure, including heart failure in individuals receiving beta-antagonist therapy.
Summary of Ischemia
Ischemia is a condition characterized by an interruption or inadequate supply of blood to tissue, which causes oxygen deprivation in the affected tissue. Myocardial ischemia is a condition caused by a blockage or constriction of one or more of the coronary arteries, such as can occur with atherosclerotic plaque occlusion or rupture. The blockade or constriction causes oxygen deprivation of the non-perfused tissue, which can cause tissue damage. Further, upon reperfusion with subsequent reoxygenation of the tissue, when the blood is able to flow again or the oxygen demand of the tissue subsides, additional injury can be caused by oxidative stress.
Ischemia/reperfusion injury refers to tissue damage caused by oxygen deprivation followed by reoxygenation. The effects of ischemia/reperfusion injury in a subject experiencing the condition can be fatal, particularly when the injury occurs in a critical organ such as the heart or brain.
Accordingly, compounds and compositions effective in preventing or protecting against ischemia/reperfusion injury would be useful pharmaceuticals. Compounds such as nitroglycerin have been used for a long period of time to help control vascular tone and protect against myocardial ischemia/reperfusion injury. It was discovered that the nitric oxide molecule was responsible for nitroglycerin's beneficial effects. This discovery prompted interest in medical uses for nitric oxide and investigations into related species such as nitroxyl. As reported in U.S. patent application Ser. No. 10/463,084 (U.S. Publication No. 2004/0038947) administration of a compound that donates nitroxyl under physiological conditions, prior to ischemia, can attenuate ischemia/reperfusion injury to tissues, for example, myocardial tissues. This beneficial effect was reported as a surprising result given that nitroxyl was previously reported to increase ischemia/reperfusion injury (See, Ma et al., “Opposite Effects of Nitric Oxide and Nitroxyl on Postischemic Myocardial Injury,” Proc. Nat'l Acad. Sci., 96(25): 14617-14622 (1999), reporting that administration of Angeli's salt (a nitroxyl donor under physiological conditions) to anesthetized rabbits during ischemia and 5 minutes prior to reperfusion increased myocardial ischemia/reperfusion injury and Takahira et al., “Dexamethasone Attenuates Neutrophil Infiltration in the Rat Kidney in Ischemia/Reperfusion Injury: The Possible Role of Nitroxyl,” Free Radical Biology & Medicine, 31(6):809-815 (2001) reporting that administration of Angeli's salt during ischemia and 5 minutes before reperfusion of rat renal tissue contributed to neutrophil infiltration into the tissue, which is believed to mediate ischemia/reperfusion injury). In particular, pre-ischemic administration of Angeli's salt and isopropylamine/NO has been reported to prevent or reduce ischemia/reperfusion injury.
Summary of Nitroxyl Donors
To date, the vast majority of studies of the biological effect of HNO have used the donor sodium dioxotrinitrate (“Angeli's salt” or “AS”). However, the chemical stability of AS has made it unsuitable to develop as a therapeutic agent. N-hydroxybenzenesulfonamide (“Piloty's acid” or “PA”) has previously been shown to be a nitroxyl donor at high ph (>9) (Bonner, F. T.; Ko, Y. Inorg. Chem. 1992, 31, 2514-2519). However, under physiological conditions, PA is a nitric oxide donor via an oxidative pathway (Zamora, R.; Grzesiok, A.; Weber, H.; Feelisch, M. Biochem. J. 1995, 312, 333-339). Thus, the physiological effects of AS and PA are not the same because AS is a nitroxyl donor under physiological conditions whereas PA is a nitric oxide donor under physiological conditions.
Although U.S. Pat. No. 6,936,639 and U.S. Publication No. 2004/0038947 describe PA as a compound that donates nitroxyl and note that other sulfohydroxamic acids and their derivatives are therefore also useful as nitroxyl donors, PA does not in fact donate significant amounts of nitroxyl under physiological conditions (See Zamora, supra).
Several substituted N-hydroxylbenzenesulfonamides have been reported as inhibitors of carbonic anhydrase, with no mention of HNO production (see, (a) Mincione, F.; Menabuoni, L.; Briganti., F; Mincione, G.; Scozzafava, A.; Supuran, C. T. J. Enzyme Inhibition 1998, 13, 267-284 and (b) Scozzafava, A.; Supuran, C. T., J. Med. Chem. 2000, 43, 3677-3687).
Significant Medical Need
Despite efforts towards the development of new therapies for the treatment of diseases and conditions such as heart failure and ischemia/reperfusion injury, there remains a significant interest in and need for additional or alternative compounds that treat or prevent the onset or severity of these and related diseases or conditions. In particular, there remains a significant medical need for alternative or additional therapies for the treatment of diseases or conditions that are responsive to nitroxyl therapy. New compounds that donate nitroxyl under physiological conditions and methods of using compounds that donate nitroxyl under physiological conditions may thus find use as therapies for treating, preventing and/or delaying the onset and/or development of diseases or conditions responsive to nitroxyl therapy, including heart disease and ischemia/reperfusion injury. Preferably, the therapeutic agents can improve the quality of life and/or prolong the survival time for patients with the disease or condition.